The present invention is related generally to composite materials. More specifically, the present invention is related to composites comprised of polymer coated highly oriented microfibers and to composites having a polymeric bulk or matrix phase reinforced with highly oriented microfibers.
Composite materials are well known, and commonly consist of a continuous, bulk or matrix phase, and a discontinuous, dispersed, fiber, or reinforcement phase. Some composites have a relatively brittle matrix and a relatively ductile or pliable reinforcement. The relatively pliable reinforcement, which can be in the form of fibers, can serve to impart toughness to the composite. Specifically, the reinforcement may inhibit crack propagation as cracks through the brittle matrix are deflected and directed along the length of the fibers. Other composites have a relatively soft matrix and a relatively rigid or strong reinforcement phase, which can include fibers. Such fibers can impart strength to the matrix, by transferring applied loads from the weak matrix to the stronger fibers.
Fibers that impart additional strength may be formed of polymers, metals, or other materials. Many materials, such as metals, have the disadvantage relatively high weight and density. Other materials, such as glass, may be inexpensive and lighter, but may wick moisture into the composite, which may make the composite unsuitable for some applications, such as marine applications. In particular, long-term submersion in water may lead to significant water uptake and decomposition, including delamination in some applications. The wicking may be caused by less than optimal adhesion between the fibers and the matrix phase, allowing moisture to be wicked in through the elongated voids formed between the fibers and the matrix. Use of inexpensive polymers, such as olefins, would be advantageous with respect to cost and weight, but known olefin fibers that are strong enough to impart the required strength to the composite may not be capable of receiving stress from the matrix, because of the low surface energy nature of known olefin fiber surfaces. Inexpensive polymer fibers such as olefin fibers may also allow wicking of moisture even though they are hydrophobic in nature.
Highly oriented ultrahigh molecular weight polyethylene fibers such as SPECTRA(copyright) (available from Allied Signal Corporation, Morristown N.J.) are available. These fibers have relatively large diameters and smooth surfaces, and are relatively expensive and are prepared by a gel spinning process followed by hot drawing. A need, therefore, exists for oriented fibers as a composite reinforcement phase having a larger surface area, providing a more optimal surface for binding to a matrix phase or to a cured polymer. What would also be advantageous are composite materials impervious to moisture, and composites utilizing reinforcements derived from common polymers, but having the strength of more expensive materials.
Composites are often opaque, either inherently due to the matrix phase properties or through the addition of pigments, or other components. Some polymers, when cast in sufficiently thin layers and not having pigment added, may be transparent or translucent. When fiber reinforcements are added to a transparent matrix phase the resulting composite is typically opaque or cloudy. What would be desirable, therefore, are fiber reinforced polymeric composites and composite articles that are transparent or translucent to visible light.
The present invention includes composite articles having a polymeric bulk or matrix phase and a polymeric reinforcement phase comprising polymeric microfibers. The microfibers can be provided by forming highly oriented, semi-crystalline, polymeric films or foams, followed by partially or totally microfibrillating the highly oriented film, thereby forming the microfibers. The microfibers thus formed may be present in free form as a pulp, as a non-woven web of entangled microfibers, and as a microfibrous article including partially and totally microfibrillated films. The polymeric reinforcement phase may comprise engineering fibers in combination with the polymeric microfibers. A preferred reinforcement is formed from polypropylene microfibers.
The matrix phase may be an elastomeric polymer in one embodiment, a thermoset polymer is another embodiment, a thermoplastic polymer in another embodiment, and a thermoplastic elastomeric polymer in yet another embodiment. A preferred matrix material in one embodiment is formed of thermoplastic, elastomeric syndiotactic polypropylene. One composite article according to the present invention is a brittle, rigid polymeric matrix having a microfibrous reinforcement phase. The microfibrous reinforcement phase can increase the toughness of the composite. The matrix phase may be either continuous or discontinuous. One discontinuous matrix phase includes numerous gas bubbles or pockets disposed within the matrix.
One article according to the present invention includes an elastomeric matrix having a stronger microfibrous reinforcement material within. The reinforcement can provide added strength and stiffness to the elastomeric matrix material. One strengthened composite material includes a transparent or translucent matrix material and a microfibrous reinforcement material having the same or similar refractive index as the matrix material, wherein the microfibrous reinforcement material is comprised of fibers small enough not to scatter light when essentially fully wetted by the matrix material. The resulting article may be strengthened by the microfibrous reinforcement material while appearing optically clear, or at least translucent. One composite article includes an elastomeric semi-syndiotactic polypropylene matrix phase, and a microfibrous reinforcement phase. Strengthened elastomeric composites may be used to form seals and gaskets.